Grease a 9x13 inch baking dish generously with butter.
Arrange half of the bread cubes in the dish. Scatter half of the blueberries over the bread. Repeat with the remaining bread and berries.
Whisk the eggs, milk, cream, granulated sugar, vanilla, and lemon zest in a large bowl until completely combined.
Pour the custard mixture evenly over the bread. Gently press down on the bread cubes with a spatula to ensure they soak up the liquid.
Cover with foil and refrigerate overnight, or for at least 4 hours.
Preheat oven to 350°F (175°C). Remove the casserole from the fridge while the oven heats.
Mix the flour, brown sugar, cinnamon, and cold butter in a small bowl using a pastry cutter or fork until crumbly.
Sprinkle the streusel topping evenly over the casserole.
Bake for 45 to 55 minutes until golden brown and puffed. If browning too quickly, cover loosely with foil.
Serve warm with maple syrup or a dusting of powdered sugar.
